Re: [PATCH v5 2/2] PR 20569, segv in follow_exec

[Luis Machado <lgustavo@codesourcery.com>]

On 10/25/2016 06:39 PM, Pedro Alves wrote:
> From: Sandra Loosemore <sandra@codesourcery.com>
>
> Changes in v5:
>
>  - Adjusted to enum-flags-fyication.
>
>  - Fixed symfile_add_flags confusions throughout.  Of note:
>
>    - Made symbol_file_add_main take a symfile_add_flags instead of a
>      'from_tty', and adjusted all callers (and some callers of
>      callers).  No longer exporting symfile_add_flags_1, which expects
>      an objfile_flags instead.
>
>    - current_inferior ()->symfile_flags no longer hacked with
>      SYMFILE_DEFER_BP_RESET.
>
>    - follow_exec uses SYMFILE_DEFER_BP_RESET again.
>
>  - Commit log simplified / updated.
>
> Tested on x86_64 Fedora 23, native and gdbserver.
>
> The following testcases make GDB crash whenever an invalid sysroot is
> provided, when GDB is unable to find a valid path to the symbol file:
>
>  gdb.base/catch-syscall.exp
>  gdb.base/execl-update-breakpoints.exp
>  gdb.base/foll-exec-mode.exp
>  gdb.base/foll-exec.exp
>  gdb.base/foll-vfork.exp
>  gdb.base/pie-execl.exp
>  gdb.multi/bkpt-multi-exec.exp
>  gdb.python/py-finish-breakpoint.exp
>  gdb.threads/execl.exp
>  gdb.threads/non-ldr-exc-1.exp
>  gdb.threads/non-ldr-exc-2.exp
>  gdb.threads/non-ldr-exc-3.exp
>  gdb.threads/non-ldr-exc-4.exp
>  gdb.threads/thread-execl.exp
>
> The immediate cause of the segv is that follow_exec is passing a NULL
> argument (the result of exec_file_find) to strlen.
>
> However, the problem is deeper than that: follow_exec simply isn't
> prepared for the case where sysroot translation fails to locate the
> new executable.  Actually all callers of exec_file_find have bugs due
> to confusion between host and target pathnames.  This commit attempts
> to fix all that.
>
> In terms of the testcases that were formerly segv'ing, GDB now prints
> a warning but continues execution of the new program, so that the
> tests now mostly FAIL instead.  You could argue the FAILs are due to a
> legitimate problem with the test environment setting up the sysroot
> translation incorrectly.
>
> A new representative test is added which exercises the ne wwarning
> code path even with native testing.
>
> gdb/ChangeLog:
> 2016-10-25  Sandra Loosemore  <sandra@codesourcery.com>
> 	    Luis Machado  <lgustavo@codesourcery.com>
> 	    Pedro Alves  <palves@redhat.com>
>
> 	PR gdb/20569
> 	* exceptions.c (exception_print_same): Moved here from exec.c.
> 	* exceptions.h (exception_print_same): Declare.
> 	* exec.h: Include "symfile-add-flags.h".
> 	(try_open_exec_file): New declaration.
> 	* exec.c (exception_print_same): Moved to exceptions.c.
> 	(try_open_exec_file): New function.
> 	(exec_file_locate_attach): Rename exec_file and full_exec_path
> 	variables to avoid confusion between target and host pathnames.
> 	Move pathname processing logic to exec_file_find.  Do not return
> 	early if pathname lookup fails; Call try_open_exec_file.
> 	* infrun.c (follow_exec): Split and rename execd_pathname variable
> 	to avoid confusion between target and host pathnames.  Warn if
> 	pathname lookup fails.  Pass target pathname to
> 	target_follow_exec, not hostpathname.  Call try_open_exec_file.
> 	* main.c (symbol_file_add_main_adapter): New function.
> 	(captured_main_1): Use it.
> 	* solib-svr4.c (open_symbol_file_object): Adjust to pass
> 	symfile_add_flags to symbol_file_add_main.
> 	* solib.c (exec_file_find): Incorporate fallback logic for relative
> 	pathnames formerly in exec_file_locate_attach.
> 	* symfile.c (symbol_file_add_main, symbol_file_add_main_1):
> 	Replace 'from_tty' parameter with a symfile_add_file.
> 	(symbol_file_command): Adjust to pass symfile_add_flags to
> 	symbol_file_add_main.
> 	* symfile.h (symbol_file_add_main): Replace 'from_tty' parameter
> 	with a symfile_add_file.
>
> gdb/testsuite/ChangeLog:
> 2016-10-25  Luis Machado  <lgustavo@codesourcery.com>
>
> 	* gdb.base/exec-invalid-sysroot.exp: New file.
> ---
>  gdb/exceptions.c                                |  18 ++++
>  gdb/exceptions.h                                |   3 +
>  gdb/exec.c                                      | 125 ++++++++++--------------
>  gdb/exec.h                                      |   8 ++
>  gdb/inferior.c                                  |   6 +-
>  gdb/infrun.c                                    |  48 +++++----
>  gdb/main.c                                      |  18 +++-
>  gdb/solib-svr4.c                                |   6 +-
>  gdb/solib.c                                     |  32 ++++--
>  gdb/symfile.c                                   |  21 ++--
>  gdb/symfile.h                                   |   3 +-
>  gdb/testsuite/gdb.base/exec-invalid-sysroot.exp |  70 +++++++++++++
>  12 files changed, 239 insertions(+), 119 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.base/exec-invalid-sysroot.exp
>
> diff --git a/gdb/exceptions.c b/gdb/exceptions.c
> index 9a10f66..9919938 100644
> --- a/gdb/exceptions.c
> +++ b/gdb/exceptions.c
> @@ -256,3 +256,21 @@ catch_errors (catch_errors_ftype *func, void *func_args, char *errstring,
>      return 0;
>    return val;
>  }
> +
> +/* See exceptions.h.  */
> +
> +int
> +exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
> +{
> +  const char *msg1 = e1.message;
> +  const char *msg2 = e2.message;
> +
> +  if (msg1 == NULL)
> +    msg1 = "";
> +  if (msg2 == NULL)
> +    msg2 = "";
> +
> +  return (e1.reason == e2.reason
> +	  && e1.error == e2.error
> +	  && strcmp (msg1, msg2) == 0);
> +}
> diff --git a/gdb/exceptions.h b/gdb/exceptions.h
> index 5711c1a..a2d39d7 100644
> --- a/gdb/exceptions.h
> +++ b/gdb/exceptions.h
> @@ -88,4 +88,7 @@ extern int catch_exceptions_with_msg (struct ui_out *uiout,
>  typedef int (catch_errors_ftype) (void *);
>  extern int catch_errors (catch_errors_ftype *, void *, char *, return_mask);
>
> +/* Compare two exception objects for print equality.  */
> +extern int exception_print_same (struct gdb_exception e1,
> +				 struct gdb_exception e2);
>  #endif
> diff --git a/gdb/exec.c b/gdb/exec.c
> index 6e2a296..eeca005 100644
> --- a/gdb/exec.c
> +++ b/gdb/exec.c
> @@ -136,73 +136,16 @@ exec_file_clear (int from_tty)
>      printf_unfiltered (_("No executable file now.\n"));
>  }
>
> -/* Returns non-zero if exceptions E1 and E2 are equal.  Returns zero
> -   otherwise.  */
> -
> -static int
> -exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
> -{
> -  const char *msg1 = e1.message;
> -  const char *msg2 = e2.message;
> -
> -  if (msg1 == NULL)
> -    msg1 = "";
> -  if (msg2 == NULL)
> -    msg2 = "";
> -
> -  return (e1.reason == e2.reason
> -	  && e1.error == e2.error
> -	  && strcmp (msg1, msg2) == 0);
> -}
> -
> -/* See gdbcore.h.  */
> +/* See exec.h.  */
>
>  void
> -exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
> +try_open_exec_file (const char *exec_file_host, struct inferior *inf,
> +		    symfile_add_flags add_flags)
>  {
> -  char *exec_file, *full_exec_path = NULL;
>    struct cleanup *old_chain;
>    struct gdb_exception prev_err = exception_none;
>
> -  /* Do nothing if we already have an executable filename.  */
> -  exec_file = (char *) get_exec_file (0);
> -  if (exec_file != NULL)
> -    return;
> -
> -  /* Try to determine a filename from the process itself.  */
> -  exec_file = target_pid_to_exec_file (pid);
> -  if (exec_file == NULL)
> -    {
> -      warning (_("No executable has been specified and target does not "
> -		 "support\n"
> -		 "determining executable automatically.  "
> -		 "Try using the \"file\" command."));
> -      return;
> -    }
> -
> -  /* If gdb_sysroot is not empty and the discovered filename
> -     is absolute then prefix the filename with gdb_sysroot.  */
> -  if (*gdb_sysroot != '\0' && IS_ABSOLUTE_PATH (exec_file))
> -    {
> -      full_exec_path = exec_file_find (exec_file, NULL);
> -      if (full_exec_path == NULL)
> -	return;
> -    }
> -  else
> -    {
> -      /* It's possible we don't have a full path, but rather just a
> -	 filename.  Some targets, such as HP-UX, don't provide the
> -	 full path, sigh.
> -
> -	 Attempt to qualify the filename against the source path.
> -	 (If that fails, we'll just fall back on the original
> -	 filename.  Not much more we can do...)  */
> -      if (!source_full_path_of (exec_file, &full_exec_path))
> -	full_exec_path = xstrdup (exec_file);
> -    }
> -
> -  old_chain = make_cleanup (xfree, full_exec_path);
> -  make_cleanup (free_current_contents, &prev_err.message);
> +  old_chain = make_cleanup (free_current_contents, &prev_err.message);
>
>    /* exec_file_attach and symbol_file_add_main may throw an error if the file
>       cannot be opened either locally or remotely.
> @@ -217,7 +160,9 @@ exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
>       errors/exceptions in the following code.  */
>    TRY
>      {
> -      exec_file_attach (full_exec_path, from_tty);
> +      /* We must do this step even if exec_file_host is NULL, so that
> +	 exec_file_attach will clear state.  */
> +      exec_file_attach (exec_file_host, add_flags & SYMFILE_VERBOSE);
>      }
>    CATCH (err, RETURN_MASK_ERROR)
>      {
> @@ -232,20 +177,58 @@ exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
>      }
>    END_CATCH
>
> -  TRY
> +  if (exec_file_host != NULL)
>      {
> -      if (defer_bp_reset)
> -	current_inferior ()->symfile_flags |= SYMFILE_DEFER_BP_RESET;
> -      symbol_file_add_main (full_exec_path, from_tty);
> +      TRY
> +	{
> +	  symbol_file_add_main (exec_file_host, add_flags);
> +	}
> +      CATCH (err, RETURN_MASK_ERROR)
> +	{
> +	  if (!exception_print_same (prev_err, err))
> +	    warning ("%s", err.message);
> +	}
> +      END_CATCH
>      }
> -  CATCH (err, RETURN_MASK_ERROR)
> +
> +  do_cleanups (old_chain);
> +}
> +
> +/* See gdbcore.h.  */
> +
> +void
> +exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
> +{
> +  char *exec_file_target, *exec_file_host;
> +  struct cleanup *old_chain;
> +  symfile_add_flags add_flags = 0;
> +
> +  /* Do nothing if we already have an executable filename.  */
> +  if (get_exec_file (0) != NULL)
> +    return;
> +
> +  /* Try to determine a filename from the process itself.  */
> +  exec_file_target = target_pid_to_exec_file (pid);
> +  if (exec_file_target == NULL)
>      {
> -      if (!exception_print_same (prev_err, err))
> -	warning ("%s", err.message);
> +      warning (_("No executable has been specified and target does not "
> +		 "support\n"
> +		 "determining executable automatically.  "
> +		 "Try using the \"file\" command."));
> +      return;
>      }
> -  END_CATCH
> -  current_inferior ()->symfile_flags &= ~SYMFILE_DEFER_BP_RESET;
>
> +  exec_file_host = exec_file_find (exec_file_target, NULL);
> +  old_chain = make_cleanup (xfree, exec_file_host);
> +
> +  if (defer_bp_reset)
> +    add_flags |= SYMFILE_DEFER_BP_RESET;
> +
> +  if (from_tty)
> +    add_flags |= SYMFILE_VERBOSE;
> +
> +  /* Attempt to open the exec file.  */
> +  try_open_exec_file (exec_file_host, current_inferior (), add_flags);
>    do_cleanups (old_chain);
>  }
>
> diff --git a/gdb/exec.h b/gdb/exec.h
> index 4044cb7..f50e9ea 100644
> --- a/gdb/exec.h
> +++ b/gdb/exec.h
> @@ -23,6 +23,7 @@
>  #include "target.h"
>  #include "progspace.h"
>  #include "memrange.h"
> +#include "symfile-add-flags.h"
>
>  struct target_section;
>  struct target_ops;
> @@ -113,4 +114,11 @@ extern void print_section_info (struct target_section_table *table,
>
>  extern void exec_close (void);
>
> +/* Helper function that attempts to open the symbol file at EXEC_FILE_HOST.
> +   If successful, it proceeds to add the symbol file as the main symbol file.
> +
> +   ADD_FLAGS is passed on to the function adding the symbol file.  */
> +extern void try_open_exec_file (const char *exec_file_host,
> +				struct inferior *inf,
> +				symfile_add_flags add_flags);
>  #endif
> diff --git a/gdb/inferior.c b/gdb/inferior.c
> index 1602483..de9e5ef 100644
> --- a/gdb/inferior.c
> +++ b/gdb/inferior.c
> @@ -851,8 +851,12 @@ add_inferior_command (char *args, int from_tty)
>    int i, copies = 1;
>    char *exec = NULL;
>    char **argv;
> +  symfile_add_flags add_flags = 0;
>    struct cleanup *old_chain = make_cleanup (null_cleanup, NULL);
>
> +  if (from_tty)
> +    add_flags |= SYMFILE_VERBOSE;
> +
>    if (args)
>      {
>        argv = gdb_buildargv (args);
> @@ -900,7 +904,7 @@ add_inferior_command (char *args, int from_tty)
>  	  switch_to_thread (null_ptid);
>
>  	  exec_file_attach (exec, from_tty);
> -	  symbol_file_add_main (exec, from_tty);
> +	  symbol_file_add_main (exec, add_flags);
>  	}
>      }
>
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index 00bba16..42a25f1 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -1077,15 +1077,17 @@ show_follow_exec_mode_string (struct ui_file *file, int from_tty,
>    fprintf_filtered (file, _("Follow exec mode is \"%s\".\n"),  value);
>  }
>
> -/* EXECD_PATHNAME is assumed to be non-NULL.  */
> +/* EXEC_FILE_TARGET is assumed to be non-NULL.  */
>
>  static void
> -follow_exec (ptid_t ptid, char *execd_pathname)
> +follow_exec (ptid_t ptid, char *exec_file_target)
>  {
>    struct thread_info *th, *tmp;
>    struct inferior *inf = current_inferior ();
>    int pid = ptid_get_pid (ptid);
>    ptid_t process_ptid;
> +  char *exec_file_host;
> +  struct cleanup *old_chain;
>
>    /* This is an exec event that we actually wish to pay attention to.
>       Refresh our symbol table to the newly exec'd program, remove any
> @@ -1155,7 +1157,7 @@ follow_exec (ptid_t ptid, char *execd_pathname)
>    process_ptid = pid_to_ptid (pid);
>    printf_unfiltered (_("%s is executing new program: %s\n"),
>  		     target_pid_to_str (process_ptid),
> -		     execd_pathname);
> +		     exec_file_target);
>
>    /* We've followed the inferior through an exec.  Therefore, the
>       inferior has essentially been killed & reborn.  */
> @@ -1164,14 +1166,17 @@ follow_exec (ptid_t ptid, char *execd_pathname)
>
>    breakpoint_init_inferior (inf_execd);
>
> -  if (*gdb_sysroot != '\0')
> -    {
> -      char *name = exec_file_find (execd_pathname, NULL);
> +  exec_file_host = exec_file_find (exec_file_target, NULL);
> +  old_chain = make_cleanup (xfree, exec_file_host);
>
> -      execd_pathname = (char *) alloca (strlen (name) + 1);
> -      strcpy (execd_pathname, name);
> -      xfree (name);
> -    }
> +  /* If we were unable to map the executable target pathname onto a host
> +     pathname, tell the user that.  Otherwise GDB's subsequent behavior
> +     is confusing.  Maybe it would even be better to stop at this point
> +     so that the user can specify a file manually before continuing.  */
> +  if (exec_file_host == NULL)
> +    warning (_("Could not load symbols for executable %s.\n"
> +	       "Do you need \"set sysroot\"?"),
> +	     exec_file_target);
>
>    /* Reset the shared library package.  This ensures that we get a
>       shlib event when the child reaches "_start", at which point the
> @@ -1193,7 +1198,7 @@ follow_exec (ptid_t ptid, char *execd_pathname)
>
>        inf = add_inferior_with_spaces ();
>        inf->pid = pid;
> -      target_follow_exec (inf, execd_pathname);
> +      target_follow_exec (inf, exec_file_target);
>
>        set_current_inferior (inf);
>        set_current_program_space (inf->pspace);
> @@ -1212,21 +1217,14 @@ follow_exec (ptid_t ptid, char *execd_pathname)
>
>    gdb_assert (current_program_space == inf->pspace);
>
> -  /* That a.out is now the one to use.  */
> -  exec_file_attach (execd_pathname, 0);
> +  /* Attempt to open the exec file.  SYMFILE_DEFER_BP_RESET is used
> +     because the proper displacement for a PIE (Position Independent
> +     Executable) main symbol file will only be computed by
> +     solib_create_inferior_hook below.  breakpoint_re_set would fail
> +     to insert the breakpoints with the zero displacement.  */
> +  try_open_exec_file (exec_file_host, inf, SYMFILE_DEFER_BP_RESET);
>
> -  /* SYMFILE_DEFER_BP_RESET is used as the proper displacement for PIE
> -     (Position Independent Executable) main symbol file will get applied by
> -     solib_create_inferior_hook below.  breakpoint_re_set would fail to insert
> -     the breakpoints with the zero displacement.  */
> -
> -  symbol_file_add (execd_pathname,
> -		   (inf->symfile_flags
> -		    | SYMFILE_MAINLINE | SYMFILE_DEFER_BP_RESET),
> -		   NULL, 0);
> -
> -  if ((inf->symfile_flags & SYMFILE_NO_READ) == 0)
> -    set_initial_language ();
> +  do_cleanups (old_chain);
>
>    /* If the target can specify a description, read it.  Must do this
>       after flipping to the new executable (because the target supplied
> diff --git a/gdb/main.c b/gdb/main.c
> index 23852e2..ca6a81e 100644
> --- a/gdb/main.c
> +++ b/gdb/main.c
> @@ -413,6 +413,20 @@ catch_command_errors_const (catch_command_errors_const_ftype *command,
>    return 1;
>  }
>
> +/* Adapter for symbol_file_add_main that translates 'from_tty' to a
> +   symfile_add_flags.  */
> +
> +static void
> +symbol_file_add_main_adapter (const char *arg, int from_tty)
> +{
> +  symfile_add_flags add_flags = 0;
> +
> +  if (from_tty)
> +    add_flags |= SYMFILE_VERBOSE;
> +
> +  symbol_file_add_main (arg, add_flags);
> +}
> +
>  /* Type of this option.  */
>  enum cmdarg_kind
>  {
> @@ -1029,7 +1043,7 @@ captured_main_1 (struct captured_main_args *context)
>           catch_command_errors returns non-zero on success!  */
>        if (catch_command_errors_const (exec_file_attach, execarg,
>  				      !batch_flag))
> -	catch_command_errors_const (symbol_file_add_main, symarg,
> +	catch_command_errors_const (symbol_file_add_main_adapter, symarg,
>  				    !batch_flag);
>      }
>    else
> @@ -1038,7 +1052,7 @@ captured_main_1 (struct captured_main_args *context)
>  	catch_command_errors_const (exec_file_attach, execarg,
>  				    !batch_flag);
>        if (symarg != NULL)
> -	catch_command_errors_const (symbol_file_add_main, symarg,
> +	catch_command_errors_const (symbol_file_add_main_adapter, symarg,
>  				    !batch_flag);
>      }
>
> diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
> index 258d7dc..0e18292 100644
> --- a/gdb/solib-svr4.c
> +++ b/gdb/solib-svr4.c
> @@ -1022,6 +1022,10 @@ open_symbol_file_object (void *from_ttyp)
>    gdb_byte *l_name_buf = (gdb_byte *) xmalloc (l_name_size);
>    struct cleanup *cleanups = make_cleanup (xfree, l_name_buf);
>    struct svr4_info *info = get_svr4_info ();
> +  symfile_add_flags add_flags = 0;
> +
> +  if (from_tty)
> +    add_flags |= SYMFILE_VERBOSE;
>
>    if (symfile_objfile)
>      if (!query (_("Attempt to reload symbols from process? ")))
> @@ -1071,7 +1075,7 @@ open_symbol_file_object (void *from_ttyp)
>      }
>
>    /* Have a pathname: read the symbol file.  */
> -  symbol_file_add_main (filename, from_tty);
> +  symbol_file_add_main (filename, add_flags);
>
>    do_cleanups (cleanups);
>    return 1;
> diff --git a/gdb/solib.c b/gdb/solib.c
> index 5067191..29b25d5 100644
> --- a/gdb/solib.c
> +++ b/gdb/solib.c
> @@ -380,21 +380,22 @@ solib_find_1 (char *in_pathname, int *fd, int is_solib)
>  /* Return the full pathname of the main executable, or NULL if not
>     found.  The returned pathname is malloc'ed and must be freed by
>     the caller.  If FD is non-NULL, *FD is set to either -1 or an open
> -   file handle for the main executable.
> -
> -   The search algorithm used is described in solib_find_1's comment
> -   above.  */
> +   file handle for the main executable.  */
>
>  char *
>  exec_file_find (char *in_pathname, int *fd)
>  {
> -  char *result = solib_find_1 (in_pathname, fd, 0);
> +  char *result;
> +  const char *fskind = effective_target_file_system_kind ();
> +
> +  if (in_pathname == NULL)
> +    return NULL;
>
> -  if (result == NULL)
> +  if (*gdb_sysroot != '\0' && IS_TARGET_ABSOLUTE_PATH (fskind, in_pathname))
>      {
> -      const char *fskind = effective_target_file_system_kind ();
> +      result = solib_find_1 (in_pathname, fd, 0);
>
> -      if (fskind == file_system_kind_dos_based)
> +      if (result == NULL && fskind == file_system_kind_dos_based)
>  	{
>  	  char *new_pathname;
>
> @@ -405,6 +406,21 @@ exec_file_find (char *in_pathname, int *fd)
>  	  result = solib_find_1 (new_pathname, fd, 0);
>  	}
>      }
> +  else
> +    {
> +      /* It's possible we don't have a full path, but rather just a
> +	 filename.  Some targets, such as HP-UX, don't provide the
> +	 full path, sigh.
> +
> +	 Attempt to qualify the filename against the source path.
> +	 (If that fails, we'll just fall back on the original
> +	 filename.  Not much more we can do...)  */
> +
> +      if (!source_full_path_of (in_pathname, &result))
> +	result = xstrdup (in_pathname);
> +      if (fd != NULL)
> +	*fd = -1;
> +    }
>
>    return result;
>  }
> diff --git a/gdb/symfile.c b/gdb/symfile.c
> index 616fef0..f524f56 100644
> --- a/gdb/symfile.c
> +++ b/gdb/symfile.c
> @@ -85,7 +85,7 @@ int readnow_symbol_files;	/* Read full symbols immediately.  */
>
>  static void load_command (char *, int);
>
> -static void symbol_file_add_main_1 (const char *args, int from_tty,
> +static void symbol_file_add_main_1 (const char *args, symfile_add_flags add_flags,
>  				    objfile_flags flags);
>
>  static void add_symbol_file_command (char *, int);
> @@ -1306,19 +1306,16 @@ symbol_file_add (const char *name, symfile_add_flags add_flags,
>     command itself.  */
>
>  void
> -symbol_file_add_main (const char *args, int from_tty)
> +symbol_file_add_main (const char *args, symfile_add_flags add_flags)
>  {
> -  symbol_file_add_main_1 (args, from_tty, 0);
> +  symbol_file_add_main_1 (args, add_flags, 0);
>  }
>
>  static void
> -symbol_file_add_main_1 (const char *args, int from_tty, objfile_flags flags)
> +symbol_file_add_main_1 (const char *args, symfile_add_flags add_flags,
> +			objfile_flags flags)
>  {
> -  symfile_add_flags add_flags = (current_inferior ()->symfile_flags
> -				 | SYMFILE_MAINLINE);
> -
> -  if (from_tty)
> -    add_flags |= SYMFILE_VERBOSE;
> +  add_flags |= current_inferior ()->symfile_flags | SYMFILE_MAINLINE;
>
>    symbol_file_add (args, add_flags, NULL, flags);
>
> @@ -1655,9 +1652,13 @@ symbol_file_command (char *args, int from_tty)
>      {
>        char **argv = gdb_buildargv (args);
>        objfile_flags flags = OBJF_USERLOADED;
> +      symfile_add_flags add_flags = 0;
>        struct cleanup *cleanups;
>        char *name = NULL;
>
> +      if (from_tty)
> +	add_flags |= SYMFILE_VERBOSE;
> +
>        cleanups = make_cleanup_freeargv (argv);
>        while (*argv != NULL)
>  	{
> @@ -1667,7 +1668,7 @@ symbol_file_command (char *args, int from_tty)
>  	    error (_("unknown option `%s'"), *argv);
>  	  else
>  	    {
> -	      symbol_file_add_main_1 (*argv, from_tty, flags);
> +	      symbol_file_add_main_1 (*argv, add_flags, flags);
>  	      name = *argv;
>  	    }
>
> diff --git a/gdb/symfile.h b/gdb/symfile.h
> index cd94d85..59952cb 100644
> --- a/gdb/symfile.h
> +++ b/gdb/symfile.h
> @@ -543,7 +543,8 @@ extern CORE_ADDR overlay_unmapped_address (CORE_ADDR, struct obj_section *);
>  extern CORE_ADDR symbol_overlayed_address (CORE_ADDR, struct obj_section *);
>
>  /* Load symbols from a file.  */
> -extern void symbol_file_add_main (const char *args, int from_tty);
> +extern void symbol_file_add_main (const char *args,
> +				  symfile_add_flags add_flags);
>
>  /* Clear GDB symbol tables.  */
>  extern void symbol_file_clear (int from_tty);
> diff --git a/gdb/testsuite/gdb.base/exec-invalid-sysroot.exp b/gdb/testsuite/gdb.base/exec-invalid-sysroot.exp
> new file mode 100644
> index 0000000..9665b5f
> --- /dev/null
> +++ b/gdb/testsuite/gdb.base/exec-invalid-sysroot.exp
> @@ -0,0 +1,70 @@
> +#   Copyright 1997-2016 Free Software Foundation, Inc.
> +
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> +
> +# This test exercises PR20569.  GDB would crash when attempting to follow
> +# an exec call when it could not resolve the path to the symbol file.
> +# This was the case when an invalid sysroot is provided.
> +
> +standard_testfile foll-exec.c
> +
> +global binfile
> +set binfile [standard_output_file "foll-exec"]
> +set testfile2 "execd-prog"
> +set srcfile2 ${testfile2}.c
> +set binfile2 [standard_output_file ${testfile2}]
> +
> +set compile_options debug
> +
> +# build the first test case
> +if  { [gdb_compile "${srcdir}/${subdir}/${srcfile2}" "${binfile2}" executable $compile_options] != "" } {
> +    untested "could not compile test program ${binfile2}"
> +    return -1
> +}
> +
> +if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable $compile_options] != "" } {
> +    untested "could not compile test program ${binfile}"
> +    return -1
> +}
> +
> +proc do_exec_sysroot_test {} {
> +    global binfile srcfile srcfile2 testfile testfile2
> +    global gdb_prompt
> +
> +    gdb_test_no_output "set sysroot /a/path/that/does/not/exist"
> +
> +    # Start the program running, and stop at main.
> +    #
> +    if ![runto_main] then {
> +	fail "Couldn't run ${testfile}"
> +	return
> +    }
> +
> +    # Verify that the system supports "catch exec".
> +    gdb_test "catch exec" "Catchpoint \[0-9\]* \\(exec\\)" "insert exec catchpoint"
> +    set test "continue to exec catchpoint"
> +    gdb_test_multiple "continue" $test {
> +	-re ".*Your system does not support this type\r\nof catchpoint.*$gdb_prompt $" {
> +	    unsupported $test
> +	    return
> +	}
> +	-re ".*Could not load symbols for executable.*$gdb_prompt $" {
> +	    pass $test
> +	}
> +    }
> +}
> +
> +# Start with a fresh gdb
> +clean_restart $binfile
> +do_exec_sysroot_test
>

Thanks for fixing the other confusion with the symbol file flags.

This version looks OK to me and matches the fixes i had for a v5.